Devices and processes for extruding sheet plastic from raw plastic material such as ingots and pellets and the like are well known in the art. Similarly, devices and processes for extruding Fresnel lenses from plastic through the use of heat and roller equipment for the extrusion process and dies for impressing the facet grooves on the face of the lens are known in the art. For this application, the term “plastic” shall include thermoplastic and thermosetting polymers. Polycarbonates are a particular group of thermoplastic polymers. Generally speaking, thermoplastics soften and melt when enough heat is applied. Polyethylene, polystyrene, polyvinyl chloride and polytetrafluoroethylene are examples of thermoplastics. A thermosetting polymer or thermosetting plastic, also referred to as a thermoset, is a polymer material that irreversibly cures. The cure may be done through heat, a chemical reaction, or irradiation. Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form. A thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. A cured thermosetting polymer is called a thermoset.
The use of Fresnel lenses for solar collectors is also known in the industry. Economic, operational, and maintenance considerations dictate that the use of a larger collector for solar collection applications is desirable. When a Fresnel lens is to be used for a solar collector, this usually dictates that the solar collector cannot be constructed of a single lens. Also, in the manufacturing of the Fresnel lens, ordinarily a size limitation is dictated by the size of the rollers that are used for the production of the Fresnel lens. This then requires that a solar collector be constructed from a composite of Fresnel lens components. It is also found that the use of a generally circular shape for a Fresnel lens is desirable for solar collector applications. Because of facet groove orientation limitations and geometric sizing and shape limitations, the manufacturing of Fresnel lens components for a generally circular solar collector is problematic.
An object of the present invention is to provide a die roller and a method for manufacturing a die roller for use in manufacturing Fresnel lens components for a generally circular solar collector that will provide for the matching of Fresnel lens facet grooves of the components with the desired overall pattern of the solar collector and will provide for economy in manufacturing, including the use of common size rollers and other common extrusion equipment and the minimization of material waste. For purposes of this application, the term “generally circular” shall be defined to include polygon shapes such as an octagonal or hexagonal shape which have sides which are roughly equidistant from the geometric center of the collector.
One of the principal reasons that Fresnel lenses have been used with very limited success for solar collectors is the extreme difficulty associated with extruding a lens without severely deforming the facets. This inherent problem in the manufacturing of Fresnel lenses is illustrated by the disclosures of U.S. Pat. No. 5,770,122 to Curchod (“Curchod”). The devices disclosed in Curchod are intended merely to improve the deformation of the facets of a linear Fresnel lens, after the lens has been extruded.
The principal factors that lead to the facet deformation which is inherent in the extrusion of Fresnel lenses include:                a. The relatively high temperature and the relatively low cooling rate of the plastics used for Fresnel lens extrusion.        b. The friction and surface bond between the die roller used for the extrusion process and the extruded lens.        c. Variation in the die facet and thus the lens facet cross sections.        d. For non-linear lenses, variation in the orientation of the die facets and thus the lens facets with respect to the direction of rotation of the die roller.        e. Non-uniform friction force and surface bond distributed on the lens facet cross sections by the die facets of the die roller.        f. Non-uniform cooling of the lens facet cross sections.        g. Variation in the optimum plastic temperature and roller temperature for different lens facet cross sections.        h. Variation in heat transfer rates for the different die facet cross sections.        i. Variation in heat transfer rates for the different lens facet cross sections.        
All the foregoing problems are applicable to extruders extruding Fresnel lenses in any configuration, including extruders for producing simple linear Fresnel lenses such as that disclosed in Curchod. The foregoing problems are even more pronounced in the production of angular lens segments for generally circular Fresnel lenses which is one of the principal reasons why Fresnel lenses have not been used successfully in solar collectors to date.
Prior U.S. Pat. Nos. 7,789,650, 7,789,651, and 7,789,652 to Johnson (collectively the “Johnson Patents”), the present inventor, disclose roller extruders for use in manufacturing angular lens segments or sub-segments for generally circular solar collectors. U.S. Pat. Nos 7,789,650 and 7,789,651 disclose extruders for use in manufacturing angular lens sub-segments, i.e. with each revolution of the die roller one or more pairs of angular lens sub-segments are produced. U.S. Pat. No. 7,789,652 discloses an extruder for use in manufacturing angular lens segments, i.e. with each revolution of the die roller one or more pairs of angular lens segments are produced. Each of these lens sub-segments or segments have curved facets so that when they are mounted together in an appropriate configuration, incident solar radiation is focused on a common focal point or focal zone. For a solar collector, an energy absorber is placed at the common focal point or focal zone. The present inventor has found that for most solar collector applications, utilization of a focal zone is preferable to attempting to confine the focused incident radiation to a common focal point. The absorber efficiency is improved and the maximum temperature experienced on the surface of the absorber is reduced by distributing the focused radiation over a larger focal zone.
The inherent advantage in the utilization of a focal zone on the solar energy absorber is that it allows the facets of a Fresnel lens, including the facets of Fresnel lens angular segments or sub-segments as disclosed in the Johnson Patents, to have some deviations from the normal optimal design. A lens does not have to have the capability of producing an image. It merely has to direct the incident solar radiation to a focal zone on an energy absorber.
For the Fresnel lens extruders disclosed in the Johnson Patents or more generally for the die roller of any extruder providing for the extrusion of Fresnel lens angular segments or sub-segments of Fresnel lens angular segments, the varying cross-section of the die facets and the corresponding lens facets or grooves, and the varying angle of each facet with respect to the line of separation of the extruded lens segment or sub-segment from the die roller results in highly variable forces being exerted on the facets as they are separated from the corresponding contact surface on the die roller. In other words, the angle that the die facets and thus the lens facets at a particular longitudinal position on the line of separation make with the line of separation varies substantially with position on the line of separation and further vary substantially at that point on the line of separation as the die roller progresses through a revolution. As stated above, because (a) the line of separation of the facets of the extruded lens from the die roller varies from being longitudinal, i.e. aligned with the axis of the roller, to being lateral, i.e. perpendicular to the axis of the roller; and (b) the cross-section of the facets varies substantially with the position from the center of curvature, the extent of the deformation caused at the moment of or subsequent to the separation of the extruded lens from a die roller will vary substantially from insignificant deformation to substantial deformation. The ability of the lens assembled from the angular lens segments or sub-segments to focus the incident solar radiation on a desired focal zone of the energy absorber may be greatly impaired by the deformation of the lens facets.
It is an objective of the present invention to provide a die roller for the manufacturing of Fresnel lens angular segments or angular lens sub-segments which minimizes lens facet distortion while minimizing deviation from optimum design and optimal cross-section for the lens facets to be extruded.
It is a further objective of the present invention to provide a die roller for the extrusion of Fresnel lens angular segments or angular sub-segments which provide for optimization of the solar collector assembled from the angular segments or sub-segments in focusing of incident solar radiation on the focal zone of an energy absorber.
It is a further objective of the present invention to provide a die roller for the extrusion of Fresnel lens angular segments or angular sub-segments which optimizes the efficiency of the solar collector assembled from the angular segments or sub-segments extruded.
It is a further objective of the present invention to provide a method for manufacturing a die roller for meeting the foregoing objectives.